The present invention is directed to resolution conversion of binary images. More specifically, the present invention is directed to mapping of binary images to high addressability binary images for simulating high resolution printing, the mapping being reversible in the form of an inverse mapping transformation so that halftone rendering is not adversely affected.
Image information, be it color or black and white, is commonly generated in a bitmap format at a particular resolution Kxc3x97Lxc3x97b, corresponding to a desired printer output, where K is a number of spots per unit of length in one dimension, L is a number of spots per unit length in the other dimension, and b is the depth of each pixel, in number of levels. This bitmap is present for every color separation of the output device; i.e., four bitmaps for a four-color output device, three bitmaps for a three-color output device, two bitmaps for a two-color output device and one bitmap for a black and white output device.
Generally, rendering high resolution images on laser printers requires both higher fast-scan addressability and slow-scan addressability. It is relatively easier to render fast-scan addressability by increasing the number of bits delivered to the laser printer in the time required for the beam to travel edge to edge. Increasing slow-scan addressability generally requires that either the polygon mirror in the printer be rotated at twice normal speed, or that the process speed be lowered by half, or, still further, by doubling the number of laser beams in the raster output scanner (ROS). Typically, these options add additional costs to the end device or adversely affect performance. Therefore, increasing slow-scan addressability is generally undesirable.
It has thus been suggested to simply convert the image to a new resolution using scaling methods. However, such a solution can cause undesirable loss of information content in the converted image, especially when the resolution of the image is greater than the printer""s resolution.
Another attempt to provide a suitable resolution conversion of binary images known as xe2x80x9cperspective projection conversionxe2x80x9d is taught in U.S. Pat. No. 5,742,708, assigned to the assignee of this application. The perspective projection conversion technique converts binary images to the printer""s resolution while retaining the information content of the original image. It simulates higher resolution by increasing fast-scan addressability and appropriately modulating the fast-scan pixels to produce the appearance of a higher resolution. Perspective projection infers 600xc3x97600xc3x978 gray pixels by counting up higher resolution binary output then performing high addressable error diffusion to render high addressable by 600 spi in the slow-scan direction. While this is advantageous, certain error diffusion processes that are typically used in conjunction with perspective projection conversion make the forward conversion process irreversible. Among other undesirable effects, this prevents designers from creating multipass halftone arrays because of certain random artifacts introduced by error diffusion.
Therefore, it is desirable to provide a resolution conversion process which converts binary images to the printer""s resolution but which is also completely reversible so that conversion between high resolution and high addressability can be performed several times during processing. More particularly, the conversion between high resolution and high addressability can be performed as many times as is necessary by the processing. As an example, repeated conversion between high resolution and high addressability is required when processing a document using a banded imaging model.
It is further desirable to provide a binary image resolution conversion system which is very simple and can be implemented in software or hardware with very little overhead. Preferably, the system simulates high resolution printing using high addressability without affecting halftone rendering.
In accordance with one aspect of the present invention, a method and apparatus are provided for simulating high resolution printing by converting high resolution bits of a binary image to sub-pixel high addressable bits by using a forward resolution conversion. A first binary image is received having a first fast-scan resolution and a first slow-scan resolution. The first binary image is converted to a second image having a second fast-scan resolution and a second slow-scan resolution. The second fast-scan resolution is greater than the first fast-scan resolution, and the second slow-scan resolution is less than the first slow-scan resolution. Preferably, the conversion of the first binary image to the second binary image is carried out by performing a series of direct mapping transformations of groups of pixel xe2x80x9ctilesxe2x80x9d from the first image to groups of pixel xe2x80x9ctilesxe2x80x9d in the second image, for the entire image so that there is a one-to-one correspondence of bits in small predefined areas of the first image to corresponding small predefined areas of the second image.
In accordance with another aspect of the invention, the mapping transformation is reversible to map a plurality of portions of the second high addressible binary image to a corresponding plurality of portions in the first high resolution binary image without loss of first or second binary image information by using an inverse resolution conversion. Preferably, the conversion of the first binary image to the second high addressability binary image includes a mapping transformation of a plurality of 2xc3x972 pixel block portions of the first image to a corresponding plurality of 4xc3x971 pixel block portions of the second image. The mapping is reversible enabling an inverse transformation of the plurality of 4xc3x971 pixel block portions of the second image to said corresponding plurality of 2xc3x972 pixel block portions of the first image.
In accordance with yet another aspect of the invention, the binary image resolution conversion by reversible direct mapping transformation enables high addressable halftone threshold arrays to be pre-processed, then rendered normally in real time at high resolution, and later forward mapped to convert the rendered image to a high addressable image suitable for marking on an image output terminal.
It is a primary advantage of the present invention that resolution conversion of binary images is enabled in a simple fashion and with a minimal software investment.
Another advantage of the present invention is that binary image resolution conversion is accomplished using a reversible direct mapping technique. This allows halftone designers to create multipass halftone arrays without undesirable artifacts introduced by error diffusion or other techniques commonly associated with prior art resolution conversion systems.
Further objects and advantages of the present invention will become apparent from the following descriptions of the various features of the present invention.